22 | SAMLEX AMERICA INC.
TABLE 7.4: STATE OF CHARGE VERSUS STANDING VOLTAGE
Percentage of
Full Charge
Standing Voltage
of Individual Cells
Standing Voltage of
12V Battery
Standing Voltage
of 24V Battery
100%
2.105V
12.63V
25.26V
90%
2.10V
12.6V
25.20V
80%
2.08V
12.5V
25.00V
70%
2.05V
12.3V
24.60V
60%
2.03V
12.2V
24.40V
50%
2.02V
12.1V
24.20V
30%
1.97V
11.8V
23.60V
20%
1.95V
11.7V
23.40V
10%
1.93V
11.6V
23.20V
0%
= / < 1.93V
= / < 11.6V
= / < 23.20V
Check the individual cell voltages / specific gravity. If the inter cell voltage difference is
more than a 0.2V, or the specific gravity difference is 0.015 or more, the cells will require
equalization. Please note that only the non-sealed / vented / flooded / wet cell batteries
are equalized. Do not equalize sealed / VRLA type of AGM or Gel Cell Batteries.
7.10 STATE OF DISCHARGE OF A LOADED BATTERY – LOW BATTERY /
DC INPUT VOLTAGE ALARM AND SHUTDOWN IN INVERTERS
Most inverter hardware estimate the State of Discharge of the loaded battery by measur-
ing the voltage at the inverter’s DC input terminals (considering that the DC input cables
are thick enough to allow a negligible voltage drop between the battery and the inverter).
Inverters are provided with a buzzer alarm to warn that the loaded battery has been
deeply discharged to around 80% of the rated capacity. Normally, the buzzer alarm is
triggered when the voltage at the DC input terminals of the inverter has dropped to
around 10.5V for a 12V battery or 21V for 24V battery at C-Rate discharge current of
C/5 Amps and electrolyte temp. of 80°F. The inverter is shut down if the terminal voltage
at C/5 discharge current falls further to 10V for 12V battery (20V for 24V battery).
The State of Discharge of a battery is estimated based on the measured terminal voltage
of the battery. The terminal voltage of the battery is dependent upon the following:
- Temperature of the battery electrolyte: Temperature of the electrolyte affects the
electrochemical reactions inside the battery and produces a Negative Voltage
Coefficient – during charging / discharging, the terminal voltage drops with rise in
temperature and rises with drop in temperature
- The amount of discharging current or “C-Rate”: A battery has non linear internal
resistance and hence, as the discharge current increases, the battery terminal voltage
decreases non-linearly
SECTION 7 |
General Information on Lead Acid
Batteries
